In older conventional feed systems for continuous digesters, high-pressure pocket feeders have been used as sluice feeders for pressurisation and transport of a chips slurry to the top of the digester.
The Handbook of Pulp, (Herbert Sixta, 2006) discloses this type of feeding with high-pressure pocket feeders (High-pressure Feeder) on page 381. The big advantage with this type of feed is that the flow of chips does not need to pass through pumps, but is instead transferred hydraulically. At the same time it is possible to maintain a high pressure in the transfer circulation to and from the digester without losing pressure. The system has however demonstrated some disadvantages in that the high-pressure pocket feeder is subjected to wear and must be adjusted so that the leakage flow from the high-pressure circulation to the low-pressure circulation is minimized. Another disadvantage is that during transfer, the temperature must be kept low so that bangs related to steam implosions do not occur in the transfer.
As early as 1957, U.S. Pat. No. 2,803,540 disclosed a feed system for a continuous chip digester where chips are pumped from an impregnation vessel to a digester in which the chips are cooked in a steam atmosphere. Here, a part of the cooking liquor is charged to the pump to obtain a pumpable consistency of 10%. However, this digester was designed for small scale production of 150-300 tons pulp per day (see col.7, r.35).
Also, U.S. Pat. No. 2,876,098 from 1959 discloses a feed system for a continuous chip digester without a high-pressure pocket feeder. Here the chips are suspended in a mixer before they are pumped with a pump to the top of the digester. The pump arrangement is provided under the digester and here the pump shaft is also fitted with a turbine in which pressurised black liquor is de-pressurised to reduce the required pump energy.
U.S. Pat. No. 3,303,088 from 1967 also discloses a feed system for a continuous chip digester without a high-pressure pocket feeder, where the wood chips are first steamed in a steaming vessel, followed by suspension of the chips in a vessel, whereafter the chips suspension is pumped to the top of the digester. U.S. Pat. No. 3,586,600 from 1971 discloses another feed system for a continuous digester mainly designed for finer wood material. Here, a high-pressure pocket feeder is not used either, and the wood material is fed with a pump 26 via an upstream impregnation vessel to the top of the digester.
Similar pumping of finer wood material to the top of a continuous digester is also disclosed in EP157279.
Typical for these embodiments of digestion systems from the late 50's to the beginning of the 70's is that these were designed for small digester houses with a limited capacity of about 100-300 tons pulp per day.
U.S. Pat. No. 5,744,004 discloses a variation of feeding wood chips into a digester where the chip mixture is fed into the digester via a series of pumps. Here, so called DISCFLO™ pumps are used. A disadvantage with this system is that this type of pump typically has a very low pump efficiency.
The previously mentioned Handbook of Pulp also discloses, on page 382, an alternative pump feed of chip mixtures called TurboFeed™. Here three pumps are used in series to feed the chips mixture to the digester. This type of feed has been patented in U.S. Pat. Nos. 5,753,075, 6,106,668, 6,325,890, 6,336,993 and 6,551,462; however in many cases, U.S. Pat. No. 3,303,088 for example, has not been taken into consideration.
U.S. Pat. No. 5,753,075 relates to pumping from a steaming vessel to a processing vessel.
U.S. Pat. No. 6,106,668 relates specifically to the addition of AQ/PS during pumping.
U.S. Pat. No. 6,325,890 relates to at least two pumps in series and the arrangement of these pumps at ground level.
U.S. Pat. No. 6,336,993 relates to a detail solution where chemicals are added to dissolve metals from the wood chips and then drawing off liquor after each pump to reduce the metal content of the pumped chips.
U.S. Pat. No. 6,551,462 essentially relates to the same system already disclosed in U.S. Pat. No. 3,303,088.
A big disadvantage with the systems with multiple pumps in series is limited accessibility. If one pump breaks down, the whole digester system stops. With 3 pumps in series and normal accessibility for each pump of 0.95, the total systems accessibility is just 0.86 (0.95*0.95*0.95=0.86).
Today's modern continuous digester houses with capacities over 4000 ton pulp per day use digesters that are 50-75 meters high and where a gauge pressure of 3-8 bar is established in the top of the digester in the case of a steam phase digester or 5-20 bar in the case of a hydraulic digester. The continuous digester systems are designed to, during the main part of operation, typically well over 80-95% of operation, run at nominal production, which makes it necessary, in regard to operational costs, for the pumps to be optimized for nominal production.
A typical digester system with a capacity of about 3000 tons with a feed system with so called “TurboFeed™” technology requires about 800 kW of pump power. It is obvious that these systems must have pumps that run at an optimized efficiency close to their nominal capacity. Such a feed system requires 19,200 kWh (800*24) per 24 hours, and at a price of 50 Euro per MWh, the operational cost comes to 960 Euro per 24 hours or 336,000 Euro per year.
The systems must also be able to guarantee operation within 50-110% of nominal production, which places great demands on the feed system. This means that a system supplier must offer pumps that are large enough to be able to handle 4000 tons, and at the same time be able to be operated within a 2000-4400 ton interval. Such a pump operated at 50% of its capacity is far from optimised, but it is necessary to at least temporarily be able to operate the pump at limited capacity in case of temporary capacity problems, for example further down the fibre line.
If this system supplier offers digester systems that can handle nominal capacities of 500-5000 tons, then the pumps must be designed in a number of different pump sizes so that each individual installation can offer, from a power and energy perspective, optimised transfer at nominal production. This makes the pumps very expensive, as normally a very limited series of pumps are manufactured in each size. To be able to meet demands of reasonably short delivery times, the system supplier must stock pumps in all pump sizes, which is very expensive.
The digester feed should also be able to guarantee optimal feeding to the top of the digester even if the flow in the transfer line is reduced to 50% of nominal flow.
This is difficult, because the flow rate in the transfer lines should be maintained above a critical level, as well-steamed chips have a tendency to sink against the direction of the transfer flow if the speed becomes too low.
A corrective measure that can be used at low rates is to increase the dilution before pumping so that a lower chip concentration is established. This is however not energy efficient as it forces the feed systems to pump unnecessarily high volumes of fluid which increases the pump energy consumption per produced unit of pulp.
Each pump has a construction point (Best Efficiency Point/“BEP”) at which the pump is intended to work. At this “BEP”, shock induced loss and frictional loss are, in the case of centrifugal pumps, at their lowest which in turn leads to that the pumps efficiency is highest at this point.
A first aim of the present invention is to provide an improved feed system for wood chips wherein optimal transfer can be achieved within a broader interval around the digesters design capacity.
Other aims of the present invention are;                improved efficiency of the feed system;        improved accessibility;        lower operational costs per pumped unit of chips;        constant chip concentration during pumping regardless of production level;        a limited range of pump sizes that can cover a broad span of the digester's production capacity;        simplified maintenance;        lower installation costs compared to feed systems with high-pressure pocket feeders or multiple pumps in series;        
The above mentioned aims may be achieved with a feed system according to the present invention.